PROJECT SUMMARY The goal of this project is to understand the aging-related genomic and functional changes in immune cells that affect responses to flu vaccination. The declining ability of the aging immune system to mount protective responses to vaccines is a major threat to the health, independence and survival of older adults. Much knowledge into the mechanisms of this decline has been gained from studies focused on one or a few immune cell subsets, or on bulk transcriptomics. However, this work has not produced two critical pieces of information: 1) an integrated view of the collective changes across relevant immune cell populations with aging, and 2) the ability to link specific immune cell subsets with their underlying cellular phenotypes/transcriptional profiles, and to compare these phenotypes and profiles as a function of age and responsiveness to vaccines. Single cell profiling, a term we use to encompass flow and mass cytometry together with single cell RNAseq (scRNAseq), is uniquely positioned to deconvolve immune system heterogeneity and identify novel distinct immune cell subsets in health and disease. Single cell profiling will therefore enable us to resolve the immune cell subset deficits relevant to the elderly immune response to vaccines from PBMCs, a highly heterogeneous starting population of cells, but one that offers the advantages of being clinically accessible, highly representative and ultimately unbiased for the purposes of data generation and analysis. We have shown that we can identify discreet cell-type-specific immune signatures of aging from PBMCs, even when such immune subsets represent a small fraction of the total PBMC pool, and have preliminary single cell profiling data from elderly PBMCs, underscoring feasibility. Here, we will analyze PBMCs at the single cell level from elderly donors before and after vaccination (Aim 1), and with or without in vitro activation of specific immune subsets (Aim 2), to understand the coordinated transcriptional and functional changes that occur, or fail to, as a function of age and vaccine responsiveness. This proposal builds on our recognized expertise in human immunology and incorporates essential expertise in cytometry and single cell transcriptomic analysis (JAX-GM), and access to elderly cohorts (George Kuchel, UConn Center on Aging). Impact: These studies will yield, with unprecedented resolution, the cell-type-specific immune signatures that distinguish responders to flu vaccine from non-responders, and will provide critical clues into the mechanisms and biomarkers of a successful vaccine immune response. Furthermore, these studies will generate a considerable amount of transcriptional and functional data related to the outputs of key innate immune and T/B-cell subsets involved in the influenza vaccine response of elderly individuals. The data will be an important resource for future studies of the elderly immune system in health and disease.